Ultrafast Optical Probes of Nonequilibrium Dynamics in Complex Matter
|Complex Adaptive Matter (CAM) embodies a
broad class of materials (hard, soft, biological) for which a combination of intrinsic
nonlinearity and feedback yields a strong sensitivity to small perturbations.
Investigation of the microscopic multiscale dynamics associated with such highly nonlinear
behavior is an essential step towards understanding the competing interactions in complex
materials. Ultrafast optical techniques are particularly well suited for investigations of
dynamical processes in materials. Moreover, there exist a community of scientists at LANL
who currently use ultrafast techniques in research spanning the disciplines of hard, soft,
and biological matter. We propose here a CAM focus area to develop and apply ultrafast
optoelectronic techniques to study nonequilibrium dynamics in CAM systems. Such a program
would build on and extend existing experimental capabilities that include femtosecond
optical pump/probe spectroscopy spanning a wide range of pump/probe energies, femtosecond
terahertz spectroscopy, and ultrafast scanning microscopy.
In addition, such a program would push the boundaries of theoretical understanding and modeling techniques for a wide range of electronic materials. Understanding the interplay of competing interactions in complex materials requires an identification of the quasiparticles involved, their internal excited states, decay channels and relaxation times. Appropriate ultrafast experiments can measure the many characteristics of such quasiparticles. Moreover, both analytical and numerical calculations required to simulate such complex interactions are easier and more accurate for short timescale dynamics than for steady-state phenomena. Such calculations can then be directly compared to experiment, allowing systematic adjustment and ultimately leading to better predictability.
Our vision for this focus area addresses key issues that span the range of hard, soft, and biological matter and that can be addressed using ultrafast optics. We expect to employ the entire range of ultrafast optoelectronic techniques in an integrated manner, as well as develop entirely new techniques. One important example of such a cross-cutting problem is the role of the polaron in CAM: its excited states, decay channels and relaxation times, and binding dynamics over a variety of complex materials and timescales. Depending on the material system examined (e.g. perovskite, polymer, organic, organic/inorganic interface) a variety of ultrafast techniques may be used. Optical pump/optical probe measurements of transient absorption or reflectivity over a wide energy range reveal the energy flow in relaxation and binding processes; optical pump/terahertz probe experiments can determine the transient mobility of the liberated charge carrier; the development of ultrafast microscopies, ultrafast nonlinear spectroscopies and ultrafast photoionization spectroscopy offers complementary probes of interface-specific dynamics. An essential element in the success of this approach is the close coupling of theory and experiment.
(Contributions to this abstract were made by Duncan McBranch, Victor Klimov, David Funk, Andy Shreve, and Alan Bishop.)